This invention relates generally to telephone circuits, and, more particularly, to telephone privacy circuits by means of which the audible ringing signal applied to a telephone may be selectively suppressed, to exclude the disturbance of incoming cells of a casual or unimportant nature while still admitting urgent calls.
For many people, the disturbance created by unwanted telephone calls cannot be avoided by answering the telephone and quickly terminating each call. Usually, a person who wishes to completely avoid disturbance will resort to one of two measures. First, the person may remove the telephone handset from its cradle, thereby causing any caller to receive a busy signal. However, this ties up a telephone line and is, accordingly, objectionable to the telephone company. Moreover, the telephone company may transmit a loud warning signal to the earpiece of the telephone as a deterrent against this privacy measure. More importantly, the person who disables his telephone in this manner is unable to receive any telephone calls at all, regardless of their urgency.
A second privacy measure is achieved by interrupting the electrical connection between the telephone receiver and the telephone line. Such an interruption may be readily effected by removing the telephone cord from its wall connection or, more conveniently, by installing an on-off switch anywhere along the telephone cord or at the telephone itself. Anyone calling such a disconnected telephone would not be aware of the disconnection and would hear a normal ringing tone. Privacy is then absolute, but important calls are never received so long as the telephone remains disconnected.
Silencing apparatus is known in the art by which the audible ringing signal is silenced by a preselected number of rings. Callers who are aware of the existence of the privacy apparatus can allow their call to "ring through" after the preselected number of rings, if they feel that their call is of sufficient urgency to interrupt the called party's privacy. However, most casual callers will hang up before the preselected number of silenced rings. In U.S. Pat. No. 3,777,069 issued in the name of Lee, ring silencing is achieved by selectively reducing the magnetomotive force applied through a ringer circuit to a value below that which is necessary to actutate the telephone bell. The circuitry of the Lee device is relatively complex and is powered by batteries.
In other patents, typified by that to Lutz et al. (U.S. Pat. No. 2,715,161), a silent ringing interval is measured by the charge or discharge time of a capacitor, and a privacy tone signal is transmitted to the caller. On receipt of the signal, the caller may either persist until the silent ringing interval has elapsed or may hang up. The Lutz system is also battery powered. Similar principles are disclosed in a patent to Friend at al. (U.S. Pat. No. 3,126,453) and Grambsch (U.S. Pat. No. 3,226,489). The Friend system also uses battery power, while the Grambsch system employs connection to 115 volt, 60 Hz ac power.
Because a telephone privacy device might be used for many hours each day, with the use of battery power, such a device would be subjected to failure at inopportune times. The requisite batteries would also result in a not insignificant cost of operation. The use of ac power in conjunction with such a privacy circuit also poses reliability problems. Disruption of the ac power supply, as in a natural disaster, when emergency telephone communication is essential, would result in loss of function of the user's telephone, which might be otherwise operational. Furthermore, the interconnection of telephone networks with ac power poses a potential safety hazard and is very strictly regulated by the Federal Communications Commission in the United States. As a practical matter, there is also a problem in positioning telephone equipment adjacent to convenient ac power outlets.
It will be apparent that privacy circuits of the prior art are relatively complex and require ancillary power sources. Alternatively, the power which a device may draw directly from the telephone line itself is strictly regulated in the United States by the Federal Communications Commission, and in many other countries by similar governmental agencies. Such devices must be designed to draw very little power both in their active and quiescent states of operation. Telephone privacy circuits of the prior art have apparently been unable to meet these requirements and there is, therefore, a significant need for a telephone privacy circuit which operates independently of ancillary power supplies, which is simple enough to be housed, if desired, in a conventional telephone, and which performs the desired privacy function. The present invention satisfies all of these requirements.